1. Field of the Invention
The present invention relates to a distance-measuring apparatus for measuring the distance to a target object using a phase difference sensor, a projecting apparatus using the distance-measuring apparatus, an imaging apparatus using the distance-measuring apparatus, and a distance-measuring method.
2. Description of the Related Art
When a projecting apparatus (projector) projects an image on a screen, the image might be distorted like a trapezoid depending on how the projector is disposed with respect to the screen.
As a method of correcting the distortion of a projected image automatically, the following is performed. The distance from a projection optical system to a screen is measured at three or more points. On the basis of a phase difference among the distances to the three points, the inclination angle of the projection plane of the screen is detected, and the distortion of an image projected on the plane is corrected (see Jpn. Pat. Appln. KOKAI Publication No. 2005-006228, for example).
Measuring the distance to a target object is called “distance measurement.” Correcting the distortion of a projected image on the basis of the result of the distance measurement is called “trapezoid correction” because the image is distorted like a trapezoid.
A projector usually includes two phase difference sensors 1 and 2 as illustrated in FIG. 19. The phase difference sensor 1 detects the inclination of a projected image in the horizontal direction, while the phase difference sensor 2 detects the inclination of a projected image in the vertical direction. The phase difference sensor 1 is so provided that its sensor components are arranged in the horizontal direction and the phase difference sensor 2 is so provided that its sensor components are arranged in the vertical direction. In FIG. 19, reference numeral 3 denotes a projector lens of the projector and reference numeral 4 denotes a display element.
With the two phase difference sensors 1 and 2, the distance in the horizontal direction can be measured at a plurality of points, as can be the distance in the vertical direction. The distortion of a projected image can thus be corrected exactly on the basis of distance data of these points.
However, the use of two phase difference sensors causes a problem of a decrease in measurement precision due to a change in the shape of the sensors as well as a problem of an increase in cost. The phase difference sensors are sometimes influenced by ambient temperature, heat of a light source, etc. and changed in shape with time. Though the change in shape is very small, the sensors have a great influence on measurement precision because the size of the sensors themselves is small.
In particular, both ends of each phase difference sensor are changed in shape and thus the precision of so-called “multi-point distance measurement” using both the ends remarkably lowers. Due to a difference in the change in shape between the two phase difference sensors, the measurement precision in the horizontal and vertical directions becomes uneven and thus the distortion of an image cannot be corrected precisely.
A technique of reducing an error in measurement using a phase difference sensor in a prior art projector is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-061925. The Publication discloses that a chart image (pattern image for distance measurement) is shifted a plurality of times and projected to measure the distance to a target object and the results of a plurality of measurements are averaged. However, the technique is a method of reducing an error by a plurality of measurements, and cannot cancel an error in measurement due to a change in the shape of the phase difference sensors and a difference between the sensors.
Jpn. Pat. Appln. KOKAI Publication No. 2005-307934 discloses a camera using phase difference sensors. The camera has a function of detecting an external factor in inhibiting measurement and a function of promoting the necessity of remeasurement. If an image is picked up with the phase difference sensors covered with an operator's finger, a warning is given to the operator. However, this warning shows only the incapability of measurement. As in the case of Publication No. 2005-061925, Publication No. 2005-307934 cannot cancel an error in measurement due to a change in the shape of the phase difference sensors and a difference between the sensors.
As described above, conventionally, the multi-point distance measurement has been achieved using two phase difference sensors. However, the use of two phase difference sensors causes the problems that the cost of the components of the sensors is increased and the measurement precision due to a difference in shape between the sensors is decreased.